The buses, which are operated by Ocean State Transit and STA of Connecticut, were observed last fall idling for excessive periods in Greenwich, Danbury, Naugatuck, Higganum and Groton, as well as in Rhode Island. In fall 2011, the EPA observed some buses idling for more than 30 minutes before the buses left the lot to pick up schoolchildren.

The EPA, which could not be reached for comment, did not specify which locations had the longest idling times.

State idling regulations, which are enforceable by the EPA, generally limit idling in Connecticut to three minutes and in Rhode Island to five minutes. Under the settlement, the companies will pay a $35,000 penalty and perform environmental projects valued at $131,000.

Greenwich Public Schools spokeswoman Kim Eves said the buses that transport students in Greenwich were found idling in the bus company's parking lot and not at any schools.

Lynette Viviani, a spokeswoman for Student Transportation of America, which is based in Wall, N.J., and owns both bus companies named in the settlement, said the company "has worked closely with the EPA and developed a comprehensive program to promote awareness and educate its drivers on the impact of idling."

Under the settlement, the parent company will implement a national training and management program to prevent excessive idling from its entire fleet of school buses. Drivers, dispatchers and managers will be trained to comply with state and local anti-idling regulations and to avoid excessive idling. The company will require all of its supervisors to monitor idling in school bus lots, will post anti-idling signs in areas where drivers congregate and will notify the school districts it serves of its anti-idling policy.

STA of Connecticut will replace nine older school buses with new buses that are equipped with state-of-the-art pollution controls. Ocean State will install GPS units and tracking systems on 117 of its buses to facilitate tracking and eliminate excessive idling.

"Our fleet of school vehicles is the one of the youngest in the industry and includes late-model buses with emissions controls serving Connecticut school districts," Viviani said. "We believe it is also incumbent upon the public to ensure all vehicles reduce their emissions as we have committed to in this agreement."

Joseph Martino, finance director for the Danbury school system, said the bus company has periodically been reminding drivers of the idling regulations.

"In colder weather, they are allowed to idle a bit longer because of problems with the diesel fuel gelling," he said.

There is also signage posted outside the schools reminding the drivers of the rule.

"In Danbury, we are fortunate because our bus fleet is one of the newer ones. Most of them are 2003-04 models," Martino said.

Idling diesel engines emit pollutants that can cause or aggravate a variety of health problems, including asthma and other respiratory diseases, and the fine particles in diesel exhaust are a likely human carcinogen, according to the EPA. Idling school buses consume about a half-gallon of fuel per hour.

The agreement is similar to earlier settlements with First Student Inc. and Durham School Services. After those companies, Student Transportation of America is the third-largest provider of school bus transportation services in North America. It operates in Rhode Island, Connecticut, Maine, New Hampshire, Vermont, New Jersey, New York, Pennsylvania, South Carolina, Florida, Illinois, Wisconsin, Minnesota, Texas, Washington and California.